metal sulfides such as zinc sulfide, cadmium sulfide, lead sulfide, copper sulfide, etc., have been widely used in the field of electronics as a display material, photoconductor material, etc., in the form of thin films or crystals. Thin films of these compounds have been made mainly by using such techniques as vacuum deposition and sputtering.
Such conventional techniques, however, have the problems that since the operations are carried out in a vacuum vessel, they are poor in productivity, can not be easily adapted to a continuous process and require very costly production equipments. Also, the size of the products obtained is is limited to the size of the vacuum vessel used, so that it is difficult to obtain a film having a large surface area. DISCLOSURE OF THE INVENTION
The purpose of the present invention is to eliminate the problems of conventional methods of forming thin films of compounds, and to this end the invention provides a process capable of forming thin films of metal sulfides in an effective and simple way.
The means for solving the problems according to the present invention comprises forming a layer of an organometallic compound having at lest one metalsulfur bond in the molecule on a substrate by printing or other methods and then thermally decomposing said organometallic compound layer in an inert gas mixed with hydrogen sulfide to thereby form a thin film of a metal sulfide.
The organometallic compounds having at least one metal-sulfur bond in the molecule which are usable in this invention include a variety of metal mercaptides and a variety of metal salts of various thiocarboxylic acids and dithiocarboxylic acids. The methods for the synthesis of these compounds are well known in the art.
The substrate used in this invention for forming thereon a layer of an organometallic compound can be optionally selected from those available in the art which can withstand the thermal decomposition temperature. Since the thermal decomposition temperature is usually around 350-450.degree. C., uncostly glass plate can be safely used as said substrate.
Said organometallic compound can be made into a uniform solution by selecting a proper solvent. This solution is coated on the substrate by known printing or coating method, and after removing the solvent by drying, the layer of said organometallic compound is thermally decomposed in an inert gas atmosphere which includes hydrogen sulfide, thereby to form a thin film of the sulfide of said metal on the substrate.
The thus produced metal sulfide, although formed at a low temperature, has the same crystal structure as the one formed at a high temperature as described in the Examples given later.
On the other hand, a salient characteristic of the metal sulfides according to the present invention is the fact that the thin film formed for such metal sulfide is an aggregate of fine particles of the compound unlike the thin films formed by the conventional methods such as vacuum deposition.
The diameter of said fine particles is subject to change according to the various conditions under which the thermal decomposition is carried out, but the result of observation by a high-resolution electron microscope showed that it was from 100 to several thousands of angstroms in an instance.
By using said means of the present invention, it is possible to form thin films of metal sulfides without using a vacuum vessel which has been a drawback to the conventional methods. Thus, the present invention can realize an improvement of productivity in the manufacture of thin films and also enables easy formation of thin films having a large area.